Method to improve ATSC-VSB transceiver performance employing a time-frequency waveform processor

ABSTRACT

A backward compatible enhancement to ATSC-VSB transmissions is provided to enable intra-field channel estimation for better channel equalization. One or more time-frequency waveforms are inserted into the transmitted signal with systematic time and frequency variation synchronized to the currently available field sync. A coherent demodulator may then directly estimate the channel frequency response. The power of the inserted waveform(s) is small compared to the existing 8-VSB signal, such that existing receivers will function as intended with only minimal degradation of performance.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention is directed, in general, to channelequalization for wireless communications and, more specifically, toproviding a time-frequency waveform for frequency domain channelestimation in a manner which allows existing receivers to continueoperating as intended.

BACKGROUND OF THE INVENTION

[0002] Within Advanced Television Standards Committee (ATSC) compliantvideo receivers, trained channel estimation is performed utilizing thefield sync available in the ATSC field. However, one trainedequalization per field is not sufficiently frequent to estimate evenslowly varying channels. Blind equalization techniques applied tointra-field channel estimation have not been entirely successful on allimportant channels. Other proposals to improve the trained equalizationupdate rate require a new training signal and are therefore not backwardcompatible with existing systems, and also improve channel estimation atthe cost of useful data rate.

[0003] One alternative to existing equalization schemes which mightimprove channel estimation without degrading the data rate is frequencydomain equalization. However, current proposals for frequency domainequalization for monocarrier systems such as vestigial sideband (VSB)modulation are essentially time-domain equalization performed in thefrequency domain. Coefficients, updates, and finite impulse response(FIR) tap calculations are performed in the frequency domain with theassistance of a fast Fourier transform (FFT) of the incoming data. Sincethe training sequence is defined only in the time domain, the errorcalculation must be performed on the filtered data afterretransformation to the time domain. The error is then retransformed tothe frequency domain for coefficient update. Similar restrictions applyto statistical (blind) and decision directed error calculations.

[0004] There is, therefore, a need in the art for improving the trainingsignal available for channel estimation. It would be desirable toprovide an enhancement to the training signal without degrading datarate and allowing existing receivers to continue to operate as intended.

SUMMARY OF THE INVENTION

[0005] To address the above-discussed deficiencies of the prior art, itis a primary object of the present invention to provide, for use in awireless communications system, a backward compatible enhancement toATSC-VSB transmissions enabling intra-field channel estimation forbetter channel equalization. One or more time-frequency waveforms areinserted into the transmitted signal with systematic time and frequencyvariation synchronized to the currently available field sync. A coherentdemodulator may then directly estimate the channel frequency response.The power of the inserted waveform(s) is small compared to the existing8-VSB signal, such that existing receivers will function as intendedwith only minimal degradation of performance.

[0006] The foregoing has outlined rather broadly the features andtechnical advantages of the present invention so that those skilled inthe art may better understand the detailed description of the inventionthat follows. Additional features and advantages of the invention willbe described hereinafter that form the subject of the claims of theinvention. Those skilled in the art will appreciate that they mayreadily use the conception and the specific embodiment disclosed as abasis for modifying or designing other structures for carrying out thesame purposes of the present invention. Those skilled in the art willalso realize that such equivalent constructions do not depart from thespirit and scope of the invention in its broadest form.

[0007] Before undertaking the DETAILED DESCRIPTION OF THE INVENTIONbelow, it may be advantageous to set forth definitions of certain wordsor phrases used throughout this patent document: the terms “include” and“comprise,” as well as derivatives thereof, mean inclusion withoutlimitation; the term “or” is inclusive, meaning and/or; the phrases“associated with” and “associated therewith,” as well as derivativesthereof, may mean to include, be included within, interconnect with,contain, be contained within, connect to or with, couple to or with, becommunicable with, cooperate with, interleave, juxtapose, be proximateto, be bound to or with, have, have a property of, or the like; and theterm “controller” means any device, system or part thereof that controlsat least one operation, whether such a device is implemented inhardware, firmware, software or some combination of at least two of thesame. It should be noted that the functionality associated with anyparticular controller may be centralized or distributed, whether locallyor remotely. Definitions for certain words and phrases are providedthroughout this patent document, and those of ordinary skill in the artwill understand that such definitions apply in many, if not most,instances to prior as well as future uses of such defined words andphrases.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] For a more complete understanding of the present invention, andthe advantages thereof, reference is now made to the followingdescriptions taken in conjunction with the accompanying drawings,wherein like numbers designate like objects, and in which:

[0009]FIG. 1 depicts a wireless communications system according to oneembodiment of the present invention;

[0010]FIG. 2 depicts in greater detail an enhanced transmitter producinga signal including a time-frequency waveform for wireless communicationsaccording to one embodiment of the present invention;

[0011]FIG. 3 depicts in greater detail an enhanced receiver utilizing asignal including a time-frequency waveform for channel estimationrelating to wireless communications according to one embodiment of thepresent invention;

[0012]FIGS. 4A and 4B are plots of simulation results for actual channelcharacteristics and estimated channel characteristics according to oneembodiment of the present invention; and

[0013]FIG. 5 is a plot of the spectral power density of thetime-frequency waveform inserted into the monocarrier signal accordingto one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0014]FIGS. 1 through 5, discussed below, and the various embodimentsused to describe the principles of the present invention in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the invention. Those skilled in the artwill understand that the principles of the present invention may beimplemented in any suitably arranged device.

[0015]FIG. 1 depicts a wireless communications system according to oneembodiment of the present invention. Wireless communications system 100includes a transmitter 101 providing enhanced signal transmission inaccordance with the present invention, with the transmitted signalincluding a time-frequency waveform as described in further detailbelow. Wireless communications system 100 also includes a receiver 102,which may be one of a plurality of such receivers, receiving the signalstransmitted by transmitter 101 and capable of demodulating the enhancedsignal, including the ability to employ the time-frequency waveform forchannel estimation in accordance with the present invention. Wirelesscommunications system 100 may optionally include one or more other“legacy” receivers 103 receiving the signals transmitted by transmitter101 and, although incapable of employing the time-frequency waveform,able to demodulate the received signal.

[0016] In the present invention, transmitter 101 and receivers 102 and103 are preferably video (including associated audio) receiversoperating according to the Advanced Television Standards Committee(ATSC) standard for vestigial sideband modulation with eight discretelevels (8-VSB). Alternatively, however, the present invention may beemployed within other wireless communications systems including voice(telephony) systems, data (e.g., Internet access) systems, or hybridvideo/voice, video/data, voice/data, or video/voice/data systems.

[0017] True frequency domain equalization is achieved in the presentinvention through the addition of pilot tones in the transmitted signal.For VSB, the additional pilot tone(s) supplement the tone (field sync)applied to assist carrier recovery. To avoid compromising the carrierrecovery function of the original pilot tone while concurrentlyproviding a better measurement of the channel characteristics, thesupplemental pilot tone is varied, preferably so that the resultingspectrum of the supplemental pilot tone is flat when measured overseveral frequency domain transform symbols. The identical frequencydistribution is systematically produced and/or replicated in both thetransmitter 101 and the receiver 102 using, for example, a linear shiftregister pseudo-random number sequence to control the frequency positionof the supplemental tone. Accordingly, a frequency sweep of the channelis achieved.

[0018]FIG. 2 depicts in greater detail an enhanced transmitter producinga signal including a time-frequency waveform for wireless communicationsaccording to one embodiment of the present invention, and is intended tobe read in conjunction with FIG. 1. Transmitter 101 includes an 8-VSBbaseband signal source 200 producing symbols for the data or informationto be transmitted in accordance with the known art, as well as an 8-VSBmodulator 201 encoding symbols by modulation of a carrier signal, alsoin accordance with the known art.

[0019] In the present invention, transmitter 101 additionally includes atime-frequency waveform generator 202, the output of which is combinedwith the output of signal source 200 by signal adder 203. The 8-VSBtransmission signal 204 output by the modulator 201 thus includesmodulation of the time-frequency waveform produced by waveform generator202 as well as the symbols produced by signal source 200.

[0020] The time-frequency waveform produced by generator 202 ispreferably a set of stepped sinusoids, each of approximately 1024samples in length with a magnitude 30 decibels (dB) below the basebandsignal. The frequency of the waveform is stepped up by a number derivedfrom the bandwidth (e.g., 6 MHz) and the per sweep resolution desired(e.g., 6 MHz/128). To achieve better per-sweep resolution, more than onesinusoid is added at different frequencies per time segment. To achievebetter intra-sweep resolution, the frequency position of the sinusoidsis continually varied from one time segment to another. In the exemplaryembodiment, the time-frequency waveforms produced by generator 202 arefrequency-hopping sinusoids with a magnitude 30 dB below the datasignal, where the frequency hops every 1024 samples (approximately 100μs) and sweeps the entire band in 128 segments. The frequency hopsfollow a predetermined sequence synchronized with the field sync signal.

[0021]FIG. 3 depicts in greater detail an enhanced receiver utilizing asignal including a time-frequency waveform for channel estimationrelating to wireless communications according to one embodiment of thepresent invention, and is intended to be read in conjunction with FIGS.1 and 2. Enhanced receiver 102 includes a conventional 8-VSB receiverfront end 300 performing such functions as carrier recovery and fieldsync detection. Enhanced receiver 102 receives and 8-VSB received signal301, which is the 8-VSB transmission signal 204 after transmissionwithin, and alteration by, the wireless channel between transmitter 101and receiver 102.

[0022] Enhanced receiver 102 also includes a conventional 8-VSB channelequalizer 302 performing channel equalization signal correction orcompensation on the output of front end 300 based upon a receivedchannel estimate, and a conventional 8-VSB receiver back end 303performing such functions as symbol decoding on the channel equalizedoutput of equalizer 302.

[0023] Receiver 102 also includes a time-frequency waveform generator304 producing an identical waveform to that produced by generator 202within transmitter 101. Generation of the predefined time-frequencywaveform is triggered by detection of a field sync signal. The initialfrequency of the sinusoids is already known by design or, alternatively,a cyclic sequence of frequencies may be employed with the initialfrequency for a particular field communicated in the reserved bits ofthe field sync signal.

[0024] The time-frequency waveform produced by generator 304 isemployed, together with data samples from front end 300, by coherentdemodulator 305 to produce a channel estimate. A set of discrete timeoscillators, one for each frequency, are initialized and the input datasamples are multiplied and integrated over the period of the timesegment. That is, a local sinusoid generator is employed to generate thetime-frequency waveform, a sample-by-sample multiplication with thereceived signal is performed, and a running sum is computed by anaccumulator. The output of the accumulator is sampled once in every timesegment (1024 samples), providing an estimate of the received amplitudeand phase of the sinusoid, and then reset. For the next time segment,the same oscillators are initialized with new sinusoids, each with a newfrequency systematically varied from the previous frequency. At the endof the sweep, the entire frequency domain characteristic of the channelis known. Because the frequency hops every 1024 samples and sweeps theentire bands in 128 segments, approximately 3 channel estimates may beperformed for a given field. The channel is thus estimated approximatelyevery 8 ms, sufficient for use with a channel changing at 50 Hz (20 msintervals) or less.

[0025] The output of the accumulator is a matched filter output directlygiving a frequency domain estimate of the channel for a single point inthe spectrum. Therefore the time series output of the accumulatordirectly samples the frequency response of the channel and no fastFourier transform (FFT) is required. However, the channel estimate maybe used directly by a frequency domain equalizer, or else transformed totime domain equalizer coefficients.

[0026] The channel estimate produced by the coherent demodulator 305 maybe further refined by the use of a channel estimator post-processor 306,which smooths the estimate, tracks time varying fades, and makes Dopplerestimates.

[0027] By proper design of the time-frequency waveform, frequentestimation of the channel is feasible utilizing simple time domainprocessing. By performing coherent demodulation at the receiver, thepower of the inserted waveform may be kept very small to permittransmission along with the currently specified system, therebymaintaining the pay load rate. By retaining the existing field sync andsynchronizing the inserted time-frequency waveform to that field sync,backward compatability with legacy receiver(s) 103 is achieved whileproviding a means to dynamically estimate the channel.

[0028]FIGS. 4A and 4B are plots of simulation results for actual channelcharacteristics and estimated channel characteristics according to oneembodiment of the present invention. FIG. 4A depicts actual channelcharacteristics, while FIG. 4B depicts a channel estimate at the outputof a coherent demodulator deriving the channel estimate from a datasignal applied to the channel utilizing the process described above.

[0029]FIG. 5 is a plot of the spectral power density of thetime-frequency waveform inserted into the monocarrier signal accordingto one embodiment of the present invention. The maximum signal power isapproximately −30 dB.

[0030] The present invention enables efficient channel decoding at highthroughput wireless communication receivers such as digital televisionreceivers, and may be incorporated into new terrestrial televisionstandards or enhancements of the existing ATSC-VSB standard.

[0031] Although the present invention has been described in detail,those skilled in the art will understand that various changes,substitutions, variations, enhancements, nuances, gradations, lesserforms, alterations, revisions, improvements and knock-offs of theinvention disclosed herein may be made without departing from the spiritand scope of the invention in its broadest form.

What is claimed is:
 1. A system for improving performance of wirelesscommunications comprising: a transmitter producing a modulated datasignal combined with one or more supplemental signals on variousfrequencies within a monocarrier channel employed to transmit themodulated data signal; and a receiver employing the one or moresupplemental signals to compute a frequency domain channel estimate foruse in equalizing the channel during demodulation of the data signal. 2.The system as set forth in claim 1 wherein the one or more supplementalsignals each employ a different frequency which changes during each of aplurality of periods, wherein the time-varying frequency for eachsupplemental signal changes from one period to a subsequent period in apredetermined sequence of frequencies within the channel.
 3. The systemas set forth in claim 2 wherein the predetermined sequence spansfrequencies within the channel to directly provide a frequency domainchannel estimate.
 4. The system as set forth in claim 2 wherein thepredetermined sequence is coordinated with a field sync within themodulated data signal.
 5. The system as set forth in claim 2 wherein theone or more supplemental signals are each transmitted with a powerselected to minimize interference with demodulation of the data signalwithout reference to the one or more supplemental signals.
 6. The systemas set forth in claim 2 wherein the time varying frequency cyclesthrough all frequencies within the predetermined sequence at a ratesufficient to permit multiple channel estimates for a single field ofthe modulated data signal.
 7. The system as set forth in claim 2 whereinthe predetermined sequence is coordinated with a field sync within themodulated data signal and wherein the one or more supplemental signalsare each transmitted with a power selected to minimize interference withdemodulation of the data signal without reference to the one or moresupplemental signals.
 8. A transmitter for improved wirelesscommunications comprising: a symbol source producing a data signal; awaveform generator producing a time-varying signal which changesfrequency during each of a plurality of periods, wherein the frequencychanges from one period to a subsequent period in a predeterminedsequence of frequencies within a channel to be employed in transmittingthe data; and a modulator producing a transmission signal from acombination of the data signal and the time-varying signal.
 9. Thetransmitter as set forth in claim 8 wherein the predetermined sequencespans the channel to directly provide a frequency domain channelestimate.
 10. The transmitter as set forth in claim 8 wherein thepredetermined sequence is coordinated with a field sync within the datasignal.
 11. The transmitter as set forth in claim 8 wherein thetime-varying signal is transmitted with a power selected to minimizeinterference with demodulation of the data signal without reference tothe time-varying signal.
 12. The transmitter as set forth in claim 8wherein the time varying signal cycles through all frequencies withinthe predetermined sequence at a rate sufficient to permit multiplechannel estimates for a single field of the data signal.
 13. Thetransmitter as set forth in claim 8 wherein the predetermined sequenceis coordinated with a field sync within the data signal and wherein thetime-varying signal is transmitted with a power selected to minimizeinterference with demodulation of the data signal without reference tothe time-varying signal.
 14. The transmitter as set forth in claim 8wherein the time-varying signal is one of a plurality of time-varyingsignals each having a different frequency during a period and eachchanging frequency from one period to a subsequent period in thepredetermined sequence of frequencies.
 15. A receiver for improvedwireless communications comprising: an equalizer performing channelequalization on a received signal utilizing a channel estimate; and acoherent demodulator producing the channel estimate from the receivedsignal and a time-varying signal corresponding to a portion of thereceived signal, wherein the time-varying signal changes frequencyduring each of a plurality of periods, wherein the frequency changesfrom one period to a subsequent period in a predetermined sequence offrequencies within a channel on which the received signal is received.16. The receiver as set forth in claim 15 further comprising: a waveformgenerator producing the time varying-signal, wherein a period durationand the predetermined sequence match a corresponding period duration andpredetermined sequence employed in generating the received signal. 17.The receiver as set forth in claim 16 wherein the waveform generatorproduces a plurality of time-varying signals each having a differentfrequency during a period and each changing frequency from one period toa subsequent period in the predetermined sequence of frequencies,wherein the coherent demodulator produces the channel estimate from thereceived signal and each of the time-varying signals.
 18. The receiveras set forth in claim 15 wherein the predetermined sequence spansfrequencies within the channel to directly provide a frequency domainchannel estimate.
 19. The receiver as set forth in claim 15 wherein thepredetermined sequence is coordinated with a field sync within thereceived signal.
 20. The receiver as set forth in claim 15 wherein thetime varying frequency cycles through all frequencies within thepredetermined sequence at a rate sufficient to permit multiple channelestimates for a single field of the received signal.
 21. The receiver asset forth in claim 15 further comprising: a channel estimatepost-processor smoothing the channel estimate, tracking time varyingfades within the channel estimate, and producing Doppler estimates forthe channel estimate.
 22. A method of wireless communication comprising:combining a data signal with one or more supplemental signals on variousfrequencies within a monocarrier channel; and employing the one or moresupplemental signals to compute a frequency domain channel estimate foruse in equalizing the channel during demodulation of the data signal.23. The method as set forth in claim 22 wherein the step of combining adata signal with one or more supplemental signals on various frequencieswithin a monocarrier channel further comprises: combining the datasignal with one or more supplemental signals each employing a differentfrequency which changes during each of a plurality of periods, whereinthe time-varying frequency for each of the supplemental signals changesfrom one period to a subsequent period in a predetermined sequence offrequencies within the channel.
 24. The method as set forth in claim 23further comprising: periodically changing a frequency for eachsupplemental signal in a predetermined sequence spanning frequencieswithin the channel to directly provide a frequency domain channelestimate.
 25. The method as set forth in claim 23 further comprising:coordinating the predetermined sequence with a field sync within thedata signal.
 26. The method as set forth in claim 23 further comprising:sweeping each supplemental signal through all frequencies within thepredetermined sequence at a rate sufficient to permit multiple channelestimates for a single field of the data signal.
 27. The method as setforth in claim 22 further comprising: providing each of the supplementalsignals with a power selected to minimize interference with demodulationof the data signal without reference to the one or more supplementalsignals.
 28. The method as set forth in claim 22 further comprising:periodically changing a frequency for each supplemental signal in apredetermined sequence of frequencies within the channel coordinatedwith a field sync within the data signal; and providing each of thesupplemental signals with a power selected to minimize interference withdemodulation of the data signal without reference to the one or moresupplemental signals.
 29. A method for improved wireless communicationscomprising: producing a data signal; producing a time-varying signalwhich changes frequency during each of a plurality of periods, whereinthe frequency changes from one period to a subsequent period in apredetermined sequence of frequencies within a channel to be employed intransmitting the data; and producing a transmission signal from acombination of the data signal and the time-varying signal.
 30. Themethod as set forth in claim 29 wherein the predetermined sequence spansthe channel to directly provide a frequency domain channel estimate. 31.The method as set forth in claim 29 wherein the predetermined sequenceis coordinated with a field sync within the data signal.
 32. The methodas set forth in claim 29 wherein the time-varying signal is providedwith a power selected to minimize interference with demodulation of thedata signal without reference to the time-varying signal.
 33. The methodas set forth in claim 29 wherein the time varying signal cycles throughall frequencies within the predetermined sequence at a rate sufficientto permit multiple channel estimates for a single field of the datasignal.
 34. The method as set forth in claim 29 wherein thepredetermined sequence is coordinated with a field sync within the datasignal and wherein the time-varying signal is transmitted with a powerselected to minimize interference with demodulation of the data signalwithout reference to the time-varying signal.
 35. The method as setforth in claim 29 wherein the time-varying signal is one of a pluralityof time-varying signals each having a different frequency during aperiod and each changing frequency from one period to a subsequentperiod in the predetermined sequence of frequencies.
 36. A method forimproved wireless communications comprising: receiving a signal;producing the channel estimate from the received signal and atime-varying signal corresponding to a portion of the received signal,wherein the time-varying signal changes frequency during each of aplurality of periods, wherein the frequency changes from one period to asubsequent period in a predetermined sequence of frequencies within achannel on which the received signal is received; and performing channelequalization on the received signal utilizing the channel estimate. 37.The method as set forth in claim 36 further comprising: producing thetime varying-signal with a period duration and the predeterminedsequence matching a corresponding period duration and predeterminedsequence employed in generating the received signal.
 38. The method asset forth in claim 37 further comprising: producing a plurality oftime-varying signals each having a different frequency during a periodand each changing frequency from one period to a subsequent period inthe predetermined sequence of frequencies, wherein the channel estimateis produced from the received signal and each of the time-varyingsignals.
 39. The method as set forth in claim 36 wherein thepredetermined sequence spans frequencies within the channel to directlyprovide a frequency domain channel estimate.
 40. The method as set forthin claim 36 wherein the predetermined sequence is coordinated with afield sync within the received signal.
 41. The method as set forth inclaim 36 wherein the time varying frequency cycles through allfrequencies within the predetermined sequence at a rate sufficient topermit multiple channel estimates for a single field of the receivedsignal.
 42. The method as set forth in claim 36 further comprising:smoothing the channel estimate, tracking time varying fades within thechannel estimate, and producing Doppler estimates for the channelestimate.
 43. A wireless communication signal comprising: a data signal;and at least one supplemental signal combined with the data signal, theat least one supplemental signal having a frequency which changes duringeach of a plurality of periods in a predetermined sequence offrequencies for a channel in which the wireless communication signal istransmitted.
 44. The wireless communications signal as set forth inclaim 43 wherein the predetermined sequence of frequencies spans thechannel.
 45. The wireless communications signal as set forth in claim 43wherein the predetermined sequence is coordinated with a field syncwithin the data signal.
 46. The wireless communications signal as setforth in claim 43 wherein at least one supplemental signal sweeps thepredetermined sequence at a rate sufficient to permit multiple channelestimates based on the at least one supplemental signal within a singlefiled of the data signal.
 47. The wireless communications signal as setforth in claim 43 wherein at least one supplemental signal has a powersufficiently less than a power for the data signal to permitdemodulation of the data signal without reference to the at least onesupplemental signal.
 48. The wireless communications signal as set forthin claim 43 wherein at least one supplemental signal further comprises:a plurality of supplemental signals each having a different frequencyduring a given period and each changing frequencies in the predeterminedsequence from one period to a subsequent period.
 49. The wirelesscommunications signal as set forth in claim 43 wherein wirelesscommunications signal is a result of modulating the combination of thedata signal and the at least one supplemental signal.